The purpose of a fastener is to clamp parts together. Engineers select fasteners, (nuts and bolts) by
how much tension or clamp force is necessary to maintain the integrity of the joint.
In the case of a bolt the tension derives from applying enough twisting force (torque) to the head to
actually stretch the bolt. In the example on the opposite page the bolt is threaded into the lower
metal piece and torque is applied until the head of the bolt is seated. After the head is seated
additional torque is applied resulting in bolt stretch. This stretch is the source of the tension which
will maintain a clamping force holding the two pieces together.
The amount of torque applied should not exceed the amount which will stretch the bolt beyond the
"elastic" region. Staying within the elastic region means that if we loosen the bolt it will return to its
original length. This is the region that engineers target when specifying installation torque values.
Stretching the bolt beyond the elastic region into the plastic region begins to permanently stretch the
bolt. If we then loosen the bolt it will not return to its original length but will have been permanently
stretched, even though it may not be visible, the bolt has been weakened. This will of course weaken
the whole assembly due to an unreliable joint.
Applying torque sufficient to stretch the bolt beyond plastic region into the failure region will cause
the bolt to break.
One way to measure fastener tension is to measure the length of the fastener after installation and
compare it to its length at rest. Since this is usually impractical, torque measurement is used as an
indication of tension.
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